She made a wish on that Ahamkara bone she carries to survive in the tunnels of Luna. In a twisted wish(of course) she became partly the thing she hates most. The Hive don't need helmets, so Eris doesn't either.

[quote]How is Eris alive for so many centuries without a ghost? How can she be on the moon's surface without a helmet? There's no atmosphere there. These 2 questions always come to mind.[/quote] When the Traveler arrived in our system; Lots of things happened. A Guardian Rises cutscene: [quote] We called it the Traveler, and its arrival changed us forever. Great cities were built on Mars and Venus. Mercury became a garden world. Human lifespan tripled. It was a time of miracles.[/quote] Golden Age grimoire card: [quote] The Traveler changed everything. It reshaped our solar system as decisively as it shattered our scientific and philosophical frameworks. To our ancestors it must have been a hammerblow - a glimpse beyond the horizon of expected possibility and into a realm of transcendent power. The Traveler kindled the Golden Age. But we built it. We remember this with pride, even after so much else has been lost. We settled our solar system and filled it with our work. Today Cryptarchs and scholars work to distill the legends of the Golden Age into truth. We know that humans lived longer, flew further, and knew more. We know that countless ancient diseases and hatreds were extinguished forever. Human aspiration gives birth to vast engineering projects, sweeping social movements, and even new forms of life. The Golden Age was not without challenges. Sources speak of internal strife, philosophical rifts- particularly around questions of machine intelligence and 'mind forking' - and enduring scientific enigmas. But humanity and its machine children tackled these problems with pride, vigor, and a contagious sense of pluralist compassion.[/quote] As for the “how can she survive on Luna?” part; Ghost Fragment: Human 4 — From the diaries of Commander Jacob Hardy, pilot, Ares One: [quote] [b]Three human beings stood on a high ridge and saw the shape of the future. Saw rain strike a millennia-old desert. Felt the air sweeten with oxygen and warm water and the beginnings of life.[/b] I am sometimes asked if I felt something die. The end of the era of human self-sufficiency. I don't know how to answer that question. I do know that I was changed. Nobody could experience that kind of wonder and remain unchanged. The decades since have proven that to me. I knew I'd never fly another mission like that. I recognized the need for a new love. That's why I threw my fresh cognitive skills into understanding the Traveler. [b]How can one entity so quickly and utterly remake an entire world? [/b]Fifty years later, I'm conversant in high mathematics, particularly topological thoughts and the slippery irreality of Light. [b]I'm involved in a project to study the Traveler's terraforming actions right now.[/b] But I still enjoy the interviews. I like going back to that mission. It makes me unspeakably happy to see how well it all turned out. And it makes me happy to remember I was there.[/quote] Many of the planets had been redefined by new elements and ways that allowed for our survival. Note how Sloane can breathe without any apparatus; even during the Red War campaign where she was Lightless. Same for Zavala during that time frame. And Asher Mir on Io. And Brother Vance on Mercury. And he isn’t even a Lightbarer. Petra our in the Tangled Shore. All of these areas had been terraformed by the Traveler to contain Oxygen, which allowed humanity to breathe.

she is a guardian without ghost but she is not a regular human, exo or awoken. she has dabbled in hive magic and could be considered something else.